Normally when you use Gmail’s “Reply” feature, your signature gets appended at the very bottom of the email, after the quoted text you’re replying to. This isn’t always the desired behavior, and many folks would rather have their signatures be included after their reply text and before the quoted text.
That is precisely what Signature Tweaks does. Another Gmail Labs feature, you enable it by clicking the Gmail Labs beaker icon, scrolling down to the Signature Tweaks line item and clicking the Enable radio button.
Next, just reply to any message and your signature file will by default be included before the quoted text instead of after. Signature Tweaks also gets rid of the separating spacer dashes that typically appear before your signature as well.